Cassini uses Pu238, which I think is ultimately derived from enriched uranium. I think the production process might be even more involved than that for the (rather different) Pu used in atomic weapons. There's nothing I know of to suggest useful concentrations of Uranium on the moon, so it makes far more sense to refine it on Earth.

200 Isp is workable for a fairly simple and reliable engine and a relatively high density propellant. A mass ratio of up to 12 (91% propellant) should be achievable, giving 5000 m/s delta-V.

Not nearly good enough. Starting from the lunar surface it takes 11,000 m/s deltaV to make a round trip to LEO, assuming aerobraking into LEO. (Recall that my transpotation system is a Moon based vehicle that makes trips to LEO and back to the moon. It meets an Earth based shuttle that just goes from Earth to LEO and back, which is a deltaV of 9,700 m/s. This splits the DeltaV almost equally between the two parts of the system.) Now if there was a water depot in LEO (mined from an asteroid, not launched from Earth). Then you would only need 5,500. Still, with the LEO depot and a LOX/LH2 engine, you could have a REALLY low mass fraction vehicle (3.4, less than 71% propellant), which could be very robust and economical.

I don't believe for a second that splitting water to oxygen and hydrogen and then liquefying them would be difficult at all. Especially with nuclear power, but it could be done with only solar power, if the production rate didn't have to be too high.

...Now if there was a water depot in LEO (mined from an asteroid, not launched from Earth). Then you would only need 5,500...

I'm assuming water depots in LEO and Lunar Orbit, (and L3 and Mars orbit eventually). So 5000 is more than enough.

campbelp2002 wrote:

...Still, with the LEO depot and a LOX/LH2 engine, you could have a REALLY low mass fraction vehicle, which could be very robust and economical.

I agree, if you can get the LOX/LH2 to LEO in the first place. Although I would much prefer a LH2 NTR with an 800+ Isp to a 450 Isp LOX/LH2.

campbelp2002 wrote:

...I don't believe for a second that splitting water to oxygen and hydrogen and then liquefying them would be difficult at all. Especially with nuclear power, but it could be done with only solar power, if the production rate didn't have to be too high.

It's not technically difficult, but it does require a LOT more equipment and power than just mining & liquefying the water in the first place does. We are talking on the order of 3 tonnes versus 100 for a pilot plant.

You also have the problem of LH2 and LOX boil off in storage. Water you can just put in the shade and let freeze.

In summary:
For the weight cost of putting a LOX/LH2 plant on the moon or an asteroid, you can put a water plant and several robot tankers, to establish all the above refueling depots.

the purpose of Uranium-enrichment and/or plutonium-production on the Moon (or - alternatively - in GEO or HEO) would be that the materials the Greens have problems with wouldn't need to be lifted from Earth in that large amounts.

The plutonium could be produced in space then and the scale of the "fabric" could be limited to thos amonts needed for Cassini-like probes.

This could be away to avoid the political problems with such probes. The equipment wouldn't have to be that large that they can produce the amounts required for electricity production for typical earthian purposes like lamps, PCs, radiators, ventilators, air conditioning, telephone etc. od billions of people.

It would be sufficient to deliver to-be-enriched-Uranium, Fluor and enrichers there. I think the launch of the enrichers would not be considered to be as dangerous as Plutonium by the Greens.

Were it desirable, I don't think uranium separation is a viable prospect in space in anything like the foreseeable future. Plutonium production seems in some ways inherently harder, there are items like reactors involved which won't scale down much. You'll miss the simple cooling you get on Earth and very many other things. There's a limited amount you can do with Pu238, it's easy to shield, but it isn't well suited to high power. IMO, large space-reactors hanging over our heads are not something to look forward to.

For the weight cost of putting a LOX/LH2 plant on the moon or an asteroid, you can put a water plant and several robot tankers, to establish all the above refueling depots.

I envision a depot where water is stored as water until it is needed, then converted at the depot to LOX and LH2. The extra equipment would be offset by a much more efficient rocket (3.4 mass fraction instead of 12). And the reactor could be much smaller, or solar could be used, since the water does not have to be converted as quickly as the vehicle's engine burns it.

Imagine this.
Many tons of water are stored at depots on the moon and in LEO. The weekly lunar shuttle arrives in LEO and fills up on LOX and LH2 that took a week to manufacture. The ship burns some of it in 10 minutes to go to the moon. 3 days later it burns the rest to land on the moon. During that 3 days the LEO depot has been making more LOX and LH2. It now has 3/7 the amount needed to fuel the vehicle. The vehicle spends 1 day on the moon for maintenance and is refueled with the LH2 and LOX that the lunar depot has been manufacturing for the last week. It then burns all (or most) of it to launch for Earth. (Aerobraking into LEO takes no propellant). After arriving in LEO 3 days later, the LEO depot now has 7/7 of the needed propellant converted and it is immediately loaded into the vehicle for the return trip. Yes, more equipment is needed at the depots, but the mass fraction of the vehicle is way better and there is nothing for the anti-nuclear people to protest, especially if the depots are solar powered. There is nothing at all that requires new technology.

By the way, I am not at all optimistic that easily accessible water will be found on any near Earth asteroid. I am a little more optimistic about the poles on the Moon, but not much. Any asteroid as close to the Sun as Mars will have had all it's water baked out millions of years ago. Comets and outer planet moons and KBOs and other things that spend most of their time in the cold depths of space far from the Sun are the only things likely to have water, and they are in orbits requiring large deltaVs to reach.

(EDIT) I have made an error. From this chart
http://www.pma.caltech.edu/~chirata/deltav.htmlit turns out that the deltaV from LEO to the Moon is 5,500 m/s but the deltaV from the Moon to LEO is only 2,300, assuming 100% aerobraking into LEO. So the LEO depot is actually more important than the lunar depot. Unfortunately it is also the least likely to be feasible, based on my doubts about the water in near Earth asteroids. So the total deltaV the Lunar vehicle needs is "only" 7,800, compared to 9,700 for the Earth shuttle. A 450 ISP vehicle that could only refuel on the Moon would need a mass fraction of 5.7, which is about 82% propellant. Not so bad.

...(EDIT) I have made an error. From this charthttp://www.pma.caltech.edu/~chirata/deltav.htmlit turns out that the deltaV from LEO to the Moon is 5,500 m/s but the deltaV from the Moon to LEO is only 2,300, assuming 100% aerobraking into LEO.

I did think your figures seemed kind of high for the return journey, but I hadn't had time to research it, and I didn't want to shoot my mouth off without a solid footing. (sorry about the mixed metaphor ) Escape velocity from the moon's surface is only 2,380 m/s (and you don't need all of that to get captured by the Earth), then its downhill all the way. I've read that a V2 rocket launched from the moon would make it to Earth.

I'm not a great fan of aerobraking, so I wonder what the delta-V for LEO insertion would be? The other 3,200 m/s or can you fudge it somewhat? I know there are some Weak Stability Boundary transfers, but they take 3 months, and I'm not sure if they work both ways.

I still much prefer NTR to chemical if you're going to go to the trouble of making LH2. Which, by the way, is not going to be easy in LEO. Liquefying H2 will require dumping a massive amount of heat, which in a vacuum can only be done with stupendously big radiators (square kilometres of them). It might very well turn out that it's a lot more efficient to make the LH2 (and LOX if we have to ) on the moon, and use much less mass and power in LEO to insulate and cool. On the moon we can cool by conduction.

Of course with water NTR you don't have these problems. You know the more I think about it, it should be possible to make a much higher Isp water NTR. Your LOX/LH2 chemical engine produces water as its exhaust, and that water (steam) is hot enough (going fast enough) to give a 450 Isp. The problem is to stop the reactor from melting. Or use a fluid core reactor.

By the way, I am not at all optimistic that easily accessible water will be found on any near Earth asteroid.

From Dr. Anthony Zuppero, a proponent of water NTR:

As much as 40% of the NEA's consist of a form of rather soft, hydrated silicate. The water content, typically ~15%, would vary between ~5% and 25% of the silicate, as a hydrated mineral of the form M * n-H2O . The hardness of the NEA dirt has been measured to be only be as hard as dried mud, unlike the sidewalk-like hardness of the rock and metal asteroids that survive reentry to the Earth's surface. Most of the water could be released by cooking the dirt at kitchen oven temperatures (~450 F). For example, a 2 km NEA would contain ~ 10,000 to 20,000 megatons of dirt. Cooking the dirt would dehydrate it, releasing ~10% of it as water and yielding ~ 1,000 Megatons of water.

As much as 40% of the NEA's consist of a form of rather soft, hydrated silicate.

Is he guessing at that or does he have some evidence? I would believe silicates but not hydrated silicates. There are no hydrated silicates on the Moon (unless the hydrogen detected at the poles is in the form of hydrated silicates). Did NEAR or any other asteroid mission detect hydrated silicates? If so, give us a link.

Escape velocity from the moon's surface is only 2,380 m/s (and you don't need all of that to get captured by the Earth), then its downhill all the way. I've read that a V2 rocket launched from the moon would make it to Earth.

I'm not a great fan of aerobraking, so I wonder what the delta-V for LEO insertion would be?

Just look at the chart. That is why I posted it! Here it is again: http://www.pma.caltech.edu/~chirata/deltav.htmlIt is 1.6 km/s to lunar orbit and another 0.7 km/s to "Earth C3=0 orbit". 1.6 + 0.7 = 2.3 km/s or 2,300 m/s, 80 m/s less than your 2,380 m/s escape velocity. So you are right, you don't need to go all the way to escape velocity. But you do need to get really close. All the rest is aerobraking, otherwise you need 0.7 km/s from C3=0 to GTO and another 2.5 from GTO to LTO, a total of 3.2 km/s. Basically, with no aerobraking, it takes the same 5,500 m/s to come back as it does to go up.

As to NTR, either water or LH2, that is not yet flight proven technology. Especially in a reusable, restartable, throttleable configuration. All the technology for LOX and LH2 production and chemical engines are in hand now, although they need extensive maturation for the kind of scenario we are considering.

You are right, it doesn't offer proof. I have not read it all yet, but near the beginning he does say this:

Quote:

Exploration of the inner solar system during the last 20 years revealed the existence of hydrated mineral objects as part of the near earth asteroids, and ice on Earth's moon, water ice moons, and ice lakes on planets and moons.

Not all of that is true. There are no "water ice moons, and ice lakes on planets and moons", in the inner solar system. Those are all in the outer (Jupiter and beyond) solar system. Ice may or may not exist on the Moon. All we know for sure is that there is hydrogen. I have no idea where the "hydrated mineral objects as part of the near earth asteroids" statement comes from. I have not seen any reports of that from NEAR or other missions, and I have looked for it, although perhaps not as extensively as I could. He may be thinking of hydrates found in some meteorites, but I don't think there is strong evidence that those meteorites came from near Earth asteroids.

As far as we know for sure right now, the only water in the inner sloar system is on Earth and Mars. In my opinion, statements that water in easily accessible form exists on near Earth asteroids is just wishful thinking not backed up by any real evidence. It is like previous statements that Mars and Venus had atmospheres that we could live in, and maybe even life. Everyone wished it were true and with no evidence to the contrary all sorts of experts said it was true. The first space craft to Mars and Venus proved them all wrong. Show me measurements from any space craft that has actually visited an asteroid that definitively shows hydrates and I will change my tune, but not before that.

The guy may be indulging in wishful thinking, or may just be a little loose with his definition of 'Inner Solar System'. Ceres, in the main belt, is now suspected to have an ice crust from Hubble observations. NEAR only got a look at one asteroid which turned out to be of the most common, non-hydrated type.

Some studies have suggested up to 25% of NEA's may be captured comets, which are definitely known to have water, before capture at least.

But I agree it's mostly speculation at this time. I think someone needs to go out and have a look! In terms of delta-V, it's much easier to get to a lot of the NEA's than the surface of the moon. Wouldn't that be a nice test mission for the CEV, without requiring a lunar lander? Put it in orbit at a few km, then dust off the old shuttle MMU 'rocket packs' and have an astronaut fly over to chip off a few samples, or even set up a small drilling rig.

As to NTR, either water or LH2, that is not yet flight proven technology. Especially in a reusable, restartable, throttleable configuration. All the technology for LOX and LH2 production and chemical engines are in hand now, although they need extensive maturation for the kind of scenario we are considering.

I might debate whether the technology for LOX and LH2 production on the moon or in orbit is in hand now, but my main point is that to explore much beyond LEO, we are going to need to go nuclear. Trying to do it purely chemical, is going to make it so difficult & so expensive, that we may doom ourselves to failure before we start.

Six months to get 6 men to Mars? Then wait 2 years to come back? At a cost of $100 Billion. Why on Earth ( ) would we do that when they had the technology back in the 60's to get 50 men there in 6 weeks? Get to Saturn in 9 months! We don't lack mature technology, we lack the political will!

Six months to get 6 men to Mars? Then wait 2 years to come back? At a cost of $100 Billion.

Well, I was thinking about only travel to the Moon. After all, this is the cisLUNAR transport topic. For Mars travel a high performance LH2 NTR would be good. But a water NTR that has ISP no better, or even LESS than LH2/LOX is definitely not worth the trouble! If LOX/LH2 can be made on the Moon, A LOX/LH2 rocket could leave from the Moon and refuel on Mars, the ONLY other place in the inner solar system we really know has water! Think of it, from a 1/6 g moon to a 1/3 g planet, both with water resources (if the Moon really does have water). It is totally awesome! Even with 450 ISP LOX/LH2 engines you would not need minimum energy trajectories. I'll have to work out what mass fractions would be needed for what transit times, but I bet it is way shorter than 6 months at pretty reasonable mass fractions. SSTM (Single Stage To Mars!)

Quote:

Wouldn't that be a nice test mission for the CEV, without requiring a lunar lander? Put it in orbit at a few km, then dust off the old shuttle MMU 'rocket packs' and have an astronaut fly over to chip off a few samples

Sounds like plan. Or even send a copy of the NEAR space craft there. Or, better yet, a private company sends a very simple robotic vehicle, developed at low cost and launched by a Falcon 9, that only detects hydrogen, no pretty pictures or other experiments. It lands there, proves the existence of hydrates or actual water and claims the asteroid!